The present invention relates to systems, apparatus and associated methods for use in the assessment of visual field functions. In its various aspects, the invention if particularly concerned with perimetry testing and visual evoked potential (VEP) testing.
Perimetry is the systematic measurement of visual field function. It is used in diagnosing different diseases in the eye, optic nerve and central nervous system. The conventional methods for assessment of visual defects of peripheral vision are based on measurement of responses to visual stimuli presented at various locations in the visual field. Several techniques use this approach:                (i) White-on-white (W-W) perimetry detects visual field impairments by measuring the sensitivity to a small luminance target presented on a homogenous background. The two most commonly used types of W-W perimetry are Goldmann kinetic perimetry and threshold static automated perimetry. With Goldmann or “kinetic” perimetry, a trained perimetrist moves the target whose brightness is held constant. The limits of the visual field are mapped for targets of different sizes and brightness. With threshold static automated perimetry, a computer program is dimmest target the patient can see at each of the test locations is found. The data are used to construct a map of the visual sensitivity of the retina.        (ii) Short wavelength automated perimetry (SWAP) utilises a blue stimulus to preferentially stimulate the blue cones. A high luminance yellow background is used to adapt to green and red cones and to saturate, simultaneously, the activity of the rods.        (iii) Frequency-doubling perimetry (FDP) uses rapidly flickering gratings. These stimuli create an illusion (apparent doubling of grating spatial frequency) that allows only a small set of retinal ganglion (M cells) cells to respond.        
These techniques reveal visual defects by comparing patients' results with those obtained with normal observers. A disadvantage of these approaches is that visual sensitivity is measured by psychophysical procedures which usually depend on the criterion used by the observers. This might result in large interindividual differences which reduce the sensitivity of the measurements.
Objective techniques have also been developed:                (i) Multifocal electoretinogram (ERG) perimetry. ERGs are electrical signals generated by retinal cells in response to a visual stimulus. MERGs are elicited by a pseudorandom binary m-sequence of luminance patches. The luminance of each sector of a dartboard-like pattern alternates between white and black. MERGs elicited by different patches are analysed by a reverse correlation technique in order to construct a map of the responses of retinal cells.        (ii) Multifocal visual evoked potential (VEP perimetry. VEPS are electrical signals generated by cortical cells in response to a visual stimulus. The stimulation is also based on a pseudo-random binary m-sequence of visual targets presented in different visual-field locations. A reverse correlation technique is used to analyse the data.        
It is known that the visual cortex has an expanded representation of the fovea because of the high density of ganglion cells in the fovea. The fovea is represented on the surface of the brain. The activity of this area can be recorded by scalp electrodes. The primary visual cortex representing the peripheral parts of the visual field, however, is folded in deeper areas of the brain. These areas of the primary visual cortex contribute little to the VEPs.
One aspect of the present invention concerns long-distance perimetry, providing new systems, apparatus and associated methods for assessment of visual-field defects which are based on measurement of long-distance interactions between an “inducing” stimulus and a “test” stimulus.
The term “long-distance interactions” usually refers to interactions between the responses to two stimuli whose separation is larger than the receptive field size. Electrophysiological studies have shown that the responses of cells in cat and monkey retina, lateral geniculate nucleus and visual cortex can be affected by a moving or shifting luminance pattern outside their receptive fields [refs.2-5]. Psychophysical data also have shown that the threshold visibility of a foveal test spot was reduced when a luminance grating is jerked in the periphery of the visual field [refs.6-8]. Measurements of visual evoked potentials (VEPs) in humans have demonstrated that the contrast reversal of a structured image reduced the magnitude of the VEPs elicited by a foveal stimulus.
One possible explanation of these findings is that long-distance interactions between the peripheral inducing stimulus and the test stimulus may increase the neural internal noise of the cells which are involved in the detection of the test pattern. The increased. internal noise will require a stronger signal in order to maintain a given level of visibility. Another possible explanation is based on the assumption that the long-distance interactions result in cortical transient-to-sustained neurone inhibition.